I am always looking to mentor motivated graduate students towards their PhD.

New! 2 Computer Engineering PhD Openings in areas of VANET-based Machine Learning, Cyber Security, and Intelligent Cognitive Radio based Networks (Open till filled)

Student Members in SNR Lab @ UML  

Current Students:

Mr. Sheikh Islam (PhD Candidate) - PhD Research Topic: "Intelligent Full-Duplex CRN Communications " (In progress)

Mr. Ibrahim Demirdogen (PhD Candidate) - PhD Research Topic: "Network Coding based Secure Cognitive Radio based Networks" (In progress)

Mr. James Bednar (MS Student) -  MS Thesis: “Systematic Design of A Blockchain-Based Continuous Authentication Mechanism Towards Vulnerable Road User (VRU) Safety” (Congratulations! James successfully passed his MS thesis defense on Nov. 20, 2023)

Mr. Shammah Mitala Kitiibwa (MS Student) -  MS Thesis: “Cybersecurity Issues and Solutions for Effective CAV Communications towards Reliable Autonomous Driving” (Congratulations! Shammah successfully passed his MS thesis defense on Dec. 19, 2019)

Mr. Vamshi Sunku Mohan Kumar (MS Student) - MS Thesis: “Blockchain based Solution for Effective CAV Communication towards Reliable Autonomous Driving” (Congratulations! Vamshi successfully passed his MS thesis defense on Dec. 19, 2019)

Mr. Xin Ma (PhD Candidate) - PhD Research Topic: "Connected Vehicle-Based Distributed Machine Learning for Adaptive Fuel Consumption Modeling and Eco-Driving" (Congratulations! Xin Ma successfully passed his PhD dissertation defense on Oct. 10, 2019, Xin joined Dell Technologies upon graduation.)

Mr. Shuai Yuan (PhD Candidate) - PhD Research Topic: " Robust Spectrum Sensing and Fairness-aware Spectrum Allocation in Emerging Heterogeneous Wireless Systems"(Congratulations! Shuai Yuan is the recipient of Dean's Scholars Award in 2020; Congratulations! Shuai Yuan successfully passed his PhD dissertation defense on Sept. 11, 2019, Shuai joined Nokia Bell Labs upon graduation.)

Ms. Fnu Poornima Manjunath (MS Student), Advance Graduate Project: “Evaluation of UAS Cyber Security Threats and Countermeasures in Surface Transportation” (Graduation date: Dec 2018. Poornima joined Cisco Systems, Inc. upon graduation.).

Mr. Lei Li (PhD Candidate) - PhD Research Topic: "Security Solution for Internet of Spectrum Device" (Congratulations! Lei Li  is the recipient of UML Outstanding PhD Graduate Student Award in Computer Engineering for 2017; Congratulations! Lei Li successfully passed his PhD dissertation defense in July, 2017. Lei joined Qualcomm Technology upon graduation.)

Mr. Huixing Zhang (MS Student) - MS Thesis: “Optimal Vehicle Control at Highway Ramps Based on Connected Vehicles and Autonomous Driving” (Co-advising with Prof. Yuanchang Xie) (Congratulations! Huixing Zhang successfully passed his MS thesis defense in July, 2014. Huixing joined NETSCOUT Systems, Inc. upon graduation.)

--------------------------------------------------

Student Members in SNR Lab @ MTU

Mr. Zhengming Li (PhD Student) - PhD Research Topic: “Balancing Security and Privacy in Vehicular Ad Hoc Networks”(Congratulations! Zhenming Li  is the recipient of  Matt Wolfe Award for Excellence in Graduate Research for 2010; Congratulations! Zhengming Li is the recipient of  the Silver student paper award at the 2010 annual meeting of the Michigan Intelligent Transportation Society;Congratulations! Zhenming Li successfully passed his PhD defense in Jan., 2012. Zhengming joined Qualcomm Technology upon graduation.)

Mr. Congyi Liu (PhD Student) - PhD Research Topic: “Robust and Effective Message Disseminations in Vehicular Ad Hoc Networks” (Congratulations!Congyi Liu is the recipient of  the Silver student paper award at the 2010 annual meeting of the Michigan Intelligent Transportation Society; Congratulations! Congyi Liu successfully passed his PhD defense in July, 2012. Congyi joined Qualcomm Technology upon graduation.)

Mr. Nathan Fettinger (MS Student) - MS Thesis: “Minimizing Distribution System Operating Costs Through Intelligently Scheduled Plug-in Hybrid Electric Vehicle” (Co-advising with Prof. Chee-Wooi Ten) (Congratulations! Nathan Fettinger successfully passed his MS thesis defense in April, 2012. Nathan joined GARMIN upon graduation.)

Mr. Neeraj H. G. Paramasivam  (MS Student) - MS Project: “Smart Grid Security"

Mr. Jitendra Bhardwaj (MS Student) - MS Project: “Light Weight Cryptographical Algorithms in Ad Hoc and Cognitive Radio Networks

Mr. Chao Zou (PhD Student) - PhD Research Topic: “Performance Analysis and Cognitive Protocol Design for Cognitive Radio Networks”(Congratulations! Chao Zou is the recipient of  Matt Wolfe Award for Excellence in Graduate Research for 2009; Congratulations! Chao is a recipient of 2010 MTU Outstanding Scholarship Award; Congratulations! Chao Zou successfully passed his PhD defense in Oct., 2010. Chao joined FIS Global)

Mr. Jialiang Li (PhD Student) - PhD Research Topic: “Modeling and Protocol Design for Mobile Mesh Network Based In-Transit Visibility (MMN-ITV) Systems”( Congratulations! Jialiang successfully passed his PhD defense in Jan. 2010; Initial Employment: Cisco Systems, Inc)

  Graduated Students:

Ms. Jing Zhong (MS Student) – MS Thesis: “Development of ns-2 based Cognitive Radio Cognitive Network Simulator”(Received degree in May 2009; Initial Employment: Qualcomm Technology)

Ms. Zhou Wang: MS Thesis: “Security Provisioning in Vehicular Ad hoc NETworks (VANETs)”(Rreceived degree in May 2007;Congratulations! Zhou Wang is the receipt of the 2005 Daimler-Chrylser Fellowship credited to her research work on Vehicular Ad Hoc Networks)(Initial Employment: Broadcom Corporation)

Mr. Vikram Oberior: MS Thesis: “Security and QoS Provisioning in Wireless Mobile Ad Hoc Networks” (received degree in July 2005)

Ms. Seetha Manickam: MS Project: “Security enhancements at MAC level for emergency applications” (received degree in May 2005)

Mr. Vijay O Pallithekethil: Defense Presentation: ”Survey on security in Mobile Ad hoc and Sensor Networks” (received degree in Dec. 2004)

Mr. Rahul Bandaru: MS Thesis: “Vulnerability Analysis and Secure Routing in Mobile Ad Hoc Networks”  (received degree in Aug. 2004)

  Other Alumni:

Mr. Tao Jin, Research Project: “Dynamic Spectrum Allocations in Cognitive Radio Networks”

Mr. Leiyuan Li, Research Project: “Power-efficient Protocol Design for Wireless Ad Hoc Networks”

Mr. Himanshu Garg, Research Project: “Design & Simulation of Routing Protocols in Wireless Sensor Networks” (Sponsored by EATON Corp.)

  Current Research Projects

1. (New!) Collaborative Research: Intelligent Full-Duplex Cognitive Radio Networks for Pervasive Heterogeneous Wireless Networking (Lead-PI: This project is sponsored by National Science Foundation through grants ECCS-2034530 & ECCS-2034616 ; In collaboration with George Mason University; Sept. 1, 2020~Aug. 31, 2024)

2. (New!) Wireless High Temperature Sensor Network for Smart Boiler Systems (Co-PI: This project is sponsored by Department of Energy  through grant DE-FE0031895; Oct., 2020 through Sept., 2023)

3. (New!) Drone-based Infrared (IR) Wireless Sensor Network for Multimodal Underground Situation Awareness (Co-PI: This project is sponsored by US Army CCDC-SC; Nov. 2020 through Nov. 2024)

4. Evaluation of UAS Cybersecurity Threats and Countermeasures in Surface Transportation (PI: This project is sponsored by Massachusetts Department of Transportation; March 12, 2018 ~ Dec. 31, 2019)

5. Security Provisioning for Cognitive Radio Networks (PI: This project is sponsored by National Science Foundation through grant CNS-1252643: February 2013~June 2016)

6. Real-time Robust and Secure Communications in Vehicular Ad Hoc Networks (VANETs) (PI:This project is sponsored by National Science Foundation through CAREER Award grant  CNS-0644056: May 2007~Sept. 2013 )

7. Modeling & Simulation of  Mobile Mesh Network based In-Transit Visibility (MMN-ITV) Systems (Co-PI: This project is sponsored by ARL and CERDEC)

8. Security Provisioning for Cognitive Radio Networks (PI: This project is sponsored by National Science Foundation through grant CNS-1017887: July 2010~February 2013)

9. Development of NS-2 Based Cognitive Radio Cognitive Network Simulator (Co-PI: This project is partially supported by CERDEC)

10. Systematic Security Design for Wireless Ad Hoc & Sensor Networks (PI: This project is partially sponsored by MTU REF Fund)

11. Cyber Security Assessment &  Vulnerability Analysis for Wireless Ad Hoc and Sensor Networks(PI: This project is sponsored by MTU REF Fund)

12. Secure Aware Cost-effective Networking Protocol Design for Wireless Ad Hoc and Sensor Networks (PI: This project is partially sponsored by MTU REF Fund)

Real-time Robust and Secure Communications in Vehicular Ad Hoc Networks (CNS-0644056)

This NSF CAREER award project is motivated by the belief that Vehicular Ad Hoc Networks (VANETs) based inter-vehicle communications could enhance traffic safety and traffic operation. VANET networks differ from general mobile ad hoc networks (MANET) because of the stringent requirements on real-time, robust and secure communications and coordination in a critical highly dynamic environment. Building on research concerning run-time static relative-position relation among neighboring vehicles, this project addresses the major challenges in access technology, dynamic power control, robust multi-hop communication, and security and privacy provisioning. The success of this project will open the door for many new applications such as:

-       Traffic safety

-       Cooperative traffic operation

-       Vehicle probe data acquisition

-       Information sharing

-       Opportunistic pervasive communications

In particular, this project will develop new approaches to access technology enabling high channel availability over dynamic multi-path wireless channel, delay-bounded dynamic power control augmenting real-time communications over high mobility, robust multi-hop message disseminations in the presence of frequent fragmentations, and security implementation promoting cooperative communication and balancing privacy and security.

Publications (student authors are in underlined bold)

1).  Z. Li, C. Liu, C. Chigan, “GPAS: A General-Purpose Automatic Survey System Based on Vehicular Ad Hoc Networks,” IEEE Wireless Communication Magazine, Vol.18 (4), August 2011.

2)      Z. Li, C. Chigan, “LEAPER: A Lightweight Reliable and Faithful Data Packet Relaying Framework for VANETs”, Elsevier Journal of Ad Hoc Networks, Vol. 9 (3), pp. 418-429, May 2011.

3)      Z. Li, C. Chigan, “RAMV: Resource-Aware Message Verification in VANETs”, Wiley Journal of Security and Communication Networks, Vol. 4 (7), pp. 771-784, July 2011.

4)      J. Li, C. Chigan, “Delay-aware Transmission Range Control for VANETs”, Proceedings of IEEE Global Communications Conference (Globecom 2010), Dec. 2010.

5)  C. Liu, C. Chigan, “Structure-less Message Aggregation (SLMA): Reliably and Efficiently Improve Information Precision and Certainty for VANETs”, Proceedings of IEEE Global Communications Conference (Globecom 2010), Dec. 2010.

6)   Z. Li, Z. Wang, and C. Chigan, “Security of Vehicular Ad Hoc Networks in Intelligent Transportation Systems,” in Wireless Technologies for Intelligent Transportation Systems, Nova Science Publishers, Inc., June 15, 2010.

7)   Z. Li, C. Chigan, “On Resource Aware Message Verification in VANETs”, Proceedings of IEEE International Conference on Communications (ICC 2010), May 2010.

8)   Z. Li, C. Chigan, and D. Wong, “AWF-NA: A Complete Solution for Tampered Packet Detection in VANETs”, Proceedings of IEEE Global Communications Conference (Globecom’08), Nov. 2008.

9)  C. Liu, C. Chigan, “RPB-MD: A Novel Robust Message Dissemination Method for VANETs”, Proceedings of IEEE Global Communications Conference (Globecom’08), Nov. 2008.

10)  C. Chigan, J. Li, “A Delay-Bounded Dynamic Interactive Power Control Algorithm for VANETs”, Proceedings of IEEE International Conference on Communications (ICC 2007), pp.5849 – 5855, June 2007.

11)  Z. Wang, C. Chigan, “Countermeasure Uncooperative Behaviors with Dynamic Trust-Token in VANETs”, Proceedings of IEEE International Conference on Communications (ICC 2007), pp.3959 – 3964, June 2007.

12)  Z. Wang, C. Chigan, “Cooperation Enhancement for Message Transmission in VANETs”, Springer Journal of Wireless Personal Communications: Special Issue on Cooperation in Wireless   Networks, Vol. 43, pp. 141-156, 2007.

13). C. Chigan, V. Oberoi, J. Li, “RPB-MACn: A Relative Position Based Collision-free MAC Nucleus for Vehicular Ad Hoc Networks”, Proceedings of IEEE Global Communications Conference (Globecom’06), pp.1-6, Nov. 2006.

Security Provisioning for Cognitive Radio Networks (CNS-1017887)

With the funding from National Science Foundation (NSF), we are investigating the networking and security provisioning issues of the emerging Cognitive Radio Networks (CRNs). Besides the QoS-aware dynamic spectrum allocation (DSA) algorithms, they proposed interference threshold based DSA algorithms, which emphasize on lowering the sensitivity of DSA to environment change by decoupling correlation of CR users. We also developed a DSA-driven MAC framework with the game theoretic DSA embedded into the MAC layer. On the security front, we comprehensively study various security aspects of CRNs. We developed a defense system against primary user emulation attack, which is based on techniques that can directly detect the activity of the primary receiver instead of the sender (causes false alarms). Our proposed DSA-based confidentiality method hides communication content via flexible dynamic spectrum access strategies, wherein both the security goal and the spectrum utility goal are taken into consideration. We also developed an NS-2 based CRN simulator. Currently, the phase I of this simulator is ready for the alpha release.

Publications (student authors are in underlined bold)

1)      C. Zou, C. Chigan, “Dynamic Spectrum Allocation based on Interference Threshold Modeling”, Proceedings of IEEE Global Communications Conference (Globecom 2010), Dec. 2010.

2)      C. Zou, C. Chigan, “Dynamic Spectrum Allocation Based Confidentiality for Cognitive Radio Networks”, Proceedings of IEEE Global Communications Conference (Globecom 2010), Dec. 2010.

3)      C. Zou, C. Chigan, “On Game Theoretic DSA-Driven MAC for Cognitive Radio Networks”, Elsevier Journal of Computer Communications, Vol. 32(18), pp.1944-1954, December, 2009.

4)      C. Zou, C. Chigan, and Z. Tian, “Development and Trends in Medium Access Control Design for Cognitive Radio Networks,” in Cognitive Radio Networks, Auerbach Publications, CRC Press 2008.

5)      C. Zou, C. Chigan, “A Game Theoretic DSA-driven MAC Framework for Cognitive Radio Networks,” IEEE International Conference on Communications (ICC 2008), May 2008

6)      C. Zou, T. Jin, C. Chigan, and Z. Tian, “QoS-Aware Distributed Spectrum Sharing for Heterogeneous Wireless Cognitive Networks”, Elsevier Journal of Computer Networks: Special Issue on Cognitive Wireless Networks, Vol. 52 (4), pp.864-878, March, 2008.

7)      T. Jin, C. Chigan, Z. Tian, “Game-theoretic Distributed Spectrum Sharing for Wireless Cognitive Networks with Heterogeneous QoS”, Proceedings of IEEE Global Communications Conference (Globecom’06), pp. 1-6, Nov. 2006.

Modeling & Simulation of  Mobile Mesh Network based In-Transit Visibility (MMN-ITV) Systems

With the funding from US Army Communications-Electronics Research Development & Engineering Center (CERDEC) and the US Army Research Lab (ARL), we have been working on energy-efficient communications of the MMN-ITV systems. It is envisioned that the MMN-ITV systems will have broad applications such as global real-time container tracking systems, supply-chain/logistic system management, and freight/package tracking. Such tracking capability is enabled by the multi-hop communications among the battery-operated container nodes in transit, and the sink nodes with satellite facilities. In this project, we employ modeling techniques to develop energy-efficient solutions for the MMN-ITV system to enable its long duration (e.g., weeks/months) of operation. We proposed a new modeling approach, pseudo-dynamic modeling, to analyze the energy-efficiency of the routing methods in the presence of the cross-MMN behavior. We also proposed a new energy-efficient routing method, energy-threshold routing, which inherits the energy-efficient properties of the shortest-path routing and the load-balanced routing synergistically. Our proposed energy-efficient neighbor discovery scheme is tailored for the MMN-ITV system to reduce the energy consumption and guarantees the success of neighbor discovery simultaneously.

Publications (student authors are in underlined bold)

1).   J. Li, C. Chigan, “On the Modeling of Energy-Efficient Routing for Mobile Mesh Network based In-Transit Visibility Systems”, IEEE Transactions on Vehicular Technology (submitted).

2)   J. Li, C. Chigan, “Energy-efficient Cooperative Neighbor Discovery in MMN-ITV Systems”, IEEE Transactions on Wireless Communications (under submission).

Systematic Security Design Framework for Wireless Ad Hoc & Sensor Networks

Albeit an appealing new wireless networking paradigm, wireless ad hoc and sensor network (WAHSN) is unfortunately inherently vulnerable and subject to a wide range of attacks. In fact, WAHSN system is not only subject to most of the well known attacks and threats that conventional wired and wireless networks suffer, but also a large number of additional attacks and threats due to the open medium, dynamically changing topology, possible node compromise, difficulty in physical protection, distributed cooperation, absence of infrastructure, lack of trust among nodes, and limited resources (processing power, storage power, memory, bandwidth and battery energy). All these factors together make WAHSN security issues non-trivial while indispensable research tasks. As a result, not only the applicability of the conventional security techniques is questionable, but also, due to its inherent resource limitation, the availability of network resources for security provisioning in WAHSN is also constrained. Moreover, mostly mission-critical applications of WAHSN further necessitate the research thrusts for WAHSN security.

This research project aims at developing a systematic design framework to efficiently provision comprehensive security services. The cores to this framework are our innovations on the resource-aware security provisioning methodology, the online dynamic trust model, the off-line trust-dependent collaborative security provisioning optimizations, and the self-adaptive security-provisioning controller. The outcome of this framework will provide a common ground for security provisioning of various WAHSN applications.

Publications (student authors are in underlined bold)

1).  C. Chigan, L. Li, Y. Ye, “A Self-adaptive Approach for Resource-aware Security Provisioning in Wireless Ad Hoc Networks”, Journal of IET Information Security (accepted).

2). C. Chigan, L. Li, Y. Ye, “Resource-aware Self-Adaptive Security Provisioning in Mobile Ad Hoc Networks”, Proceedings of IEEE Wireless Communication and Networking Conference (WCNC’05), Vol.4, pp.2118-2124, March 2005.

3).  C. Chigan, Y. Ye, L. Li, “Balancing Security against Performance in Mobile Ad Hoc Networks”, Proceedings of IEEE Vehicular Technology Conference (VTC2004-Fall), Vol.7, pp.4735-4739, Sep. 2004.

Cyber Security Assessment & Vulnerability Analysis for Wireless Ad Hoc and Sensor Networks

To cost effectively provision transparent security solution to Wireless Ad Hoc and Sensor Networks for various applications. It is essential to investigate the attacks/threats profile for the common and specific WAHSN applications. Therefore the most popular security flaws can be protected with the minimum cost (in terms of network performance degradation, and the security mechanism implementation capital). We systematically collect and categorize the security flaws of wireless ad hoc and sensor networks into node- and link-vulnerability-realm, as well as the protocol-layer-vulnerability-realm. In this project, we also thrust to develop generic cyber security assessment models to quantitatively evaluate the vulnerability and security of the information system.

Publications (student authors are in underlined bold)

1). H. Zhu, C. Chigan, F. Bao, “Turing Assessor: A New Tool for Cyber Security Quantification”, Proceedings of IEEE Wireless Communications and Networking Conference (WCNC’06), Vol. 2, pp. 629 – 633, April 2006.

2). Mr. Rahul Bandaru: MS Thesis: “Vulnerability Analysis and Secure Routing in Mobile Ad Hoc Networks”, Aug. 2004, MTU.

Secure Aware Cost-effective Networking Protocol Design for Wireless Ad Hoc and Sensor Networks

Although security issues have drawn extensive attention in the WAHSN research community, the design complexity and challenges faced by the networking protocols alone results most of the research activities focusing on the protocol development and other core functionality provisioning. Therefore, sequential security enhancements to the well accepted existing WAHSN networking protocols are necessary if the secure WAHSN applications are demanded. Among all the networking protocols, the routing protocol is of extreme importance to the security of the entire WAHSN network; otherwise, the attacker might take advantage of an insecure routing protocol and exchange the misleading routing information to any other nodes, thus the attacker has the opportunity to affect the forwarding of packets over, the entire network.

Theoretically, the list of all kinds of attacks for WAHSN could go on forever. More importantly, each security countermeasure introduced consumes certain amount of WAHSN resource, including processing power, storage power, bandwidth and energy. Therefore, a cost-effective transparent security solution for various WAHSN applications will involve: 1) carefully selected security features (confidentiality, integrity, authentication, non-repudiation) which are immune from the most threatening attacks related to the specific applications, 2) the low-end (in terms of implementation cost capital) security mechanisms which provision the application dependent security features, 3) optimality of security strength and core network performance tradeoff such that the right balance between seamless system behavior and the need to alert and protect users to potential security threats can be achieved.

Publications (student authors are in underlined bold)

1).  C. Zou, C. Chigan, “On Anonymous On-Demand Source Routing in MANETs ”, Wiley’s Journal of Security and Communication Networks, Vol. 2 (6), pp. 476–491, Nov/Dec, 2009.

2)   T. Jin, C. Chigan, “A Security-enabled Wireless Token Cluster MAC Protocol with Intelligent Token Policy”, IEEE Military Communication Conference (MILCOM 2007), pp. 1-7, Orlando, Oct. 2007.

3).  H. Zhu, F. Bao, C. Chigan, “Compact Routing Discovery Protocol with Lower Communication Complexity”, Proceedings of IEEE Wireless Communications and Networking Conference (WCNC’06), Vol. 1, pp.307 – 310, April 2006.

4).  V. Oberoi, C. Chigan, “A Generic Packet-Dropping Detection Mechanism Augmented with Efficient Power Saving in Ad Hoc Networks”, Proceedings of SPIE Symposium on Defense and Security, pp.1-10, March 2006.

5).  R. Bandaru, C. Chigan, “DAHA: A Novel Mechanism for Wormhole Attack Detection in Wireless Ad Hoc Networks”, Proceedings of SPIE Symposium on Defense and Security, pp. 1-11, March 2006.

6).  L. Li, C. Chigan, “Token Routing: A Power Efficient Method for Securing AODV Routing Protocol”, Proceedings of the 2006 IEEE International Conference On Networking, Sensing and Control (ICNSC’06), pp. 29-34, April 2006.

7).  C. Chigan, V. Oberoi, “SPAR: A Security- and Power- aware Co-design Approach for Wireless Ad Hoc Networks,” SPIE Proceedings of Symposium on Sensors, and Command, Control,  Communications, and Intelligence (C3I) Technologies for Homeland Security and Homeland, Vol. 5778, pp. 863-872 (2005).

8).  C. Chigan, L. Li, “SARA: a Self-adaptive and Resource-aware Approach to Secure Wireless Ad Hoc and Sensor Networks”, SPIE Proceedings of Symposium on Sensors, and Command, Control, Communications, and Intelligence (C3I) Technologies for Homeland Security and Homeland, Vol. 5778, pp. 833-842 (2005).

9).  C. Chigan, R. Bandaru, “Secure Nodes Misbehavior in Mobile Ad Hoc Networks”, Proceedings of IEEE Vehicular Technology Conference (VTC2004-Fall), Vol. 7, pp.4730-4734, Sep. 2004.

10). C. Chigan, L. Li, R. Bandaru, “Providing Unified Security Mechanisms for MANET Network Layer”, Proceedings of the 2004 International Conference on Pervasive Computing and Communications (PCC'04), Vol. 2, pp. 721-724, June 2004.

11).C. Chigan, R. Bandaru, “Towards A Perfect Secure Routing Protocol for Mobile Ad Hoc Networks”, Proceedings of the 2004 International Conference on Security and Management (SAM'04), pp.135-140, June 2004.